Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 2,552.1	$ 2,574.3	$ 2,578.3
Less Investment Expenses									29.5	26.9	26.9
Less Investment Income on PFA Assets									15.7	16.1	17.4
Less Amortization of Tax Credit Partnerships									14.8	16.1	14.4
Net Investment Income	629.4	615.5	626.1	621.1	643.0	619.2	633.5	619.5	2,492.1	2,515.2	2,519.6
Fixed Income Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									2,371.6	2,404.0	2,425.2
Derivative [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									35.2	28.9	22.9
Mortgage Receivable [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									109.2	107.1	100.1
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									15.7	14.8	14.1
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									18.0	15.2	13.1
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 2.4	$ 4.3	$ 2.9